One less victim of what? Torture, or the thing torture is preventing? Anyway, the answer is trivial: you'd decide the same way you decide anything else with a marginal cost and a marginal benefit. If the country has to torture one person for a day to save a thousand people fifty quality-adjusted life-years each, it seems like an obvious win. OTOH, if it's five million people for one QALR, we've wasted far more years in torture than we've saved, and we'd have drawn the line somewhere lower.